Various devices and systems are known that lift and carry workpieces from one station to another, such as endless chain and belt conveyors and shuttle-type transfer mechanisms. In multiple station transfer lines with multiple work stations spaced along the line, workpieces are typically received on locators therein and a transfer shuttle mechanism is provided for lifting and carrying workpieces from one station to another. Usually, the workpiece carrier or "live" rails of the shuttle travel in a closed loop rectangular travel path disposed in a vertical plane with the lower horizontal or retraction path leg or run oriented to underlie all of the workpieces. The entire shuttle is raised and lowered generally vertically in this plane by suitable elevator mechanisms actuated by a drive mechanism which in operation raises the shuttle to lift the workpieces on the live rails generally vertically above the stationary locators, whereupon the shuttle live rails are actuated by a synchronized horizontal reciprocating drive mechanism to advance the workpieces along a horizontal run of the travel path of the shuttle. Then the elevators are lowered to move the entire shuttle generally vertically downwardly to deposit the advanced workpieces individually in the next successive work stations. After such workpiece placement the elevators further lower the shuttle sufficiently to disengage and clear the workpieces, and the live rails are then retracted along the lower horizontal run of their travel path to reposition the shuttle for the next transfer cycle.
Such shuttle-type transfer mechanisms are preferred, and indeed required, in many processing lines over endless belt, chain or other type systems for conveying workpieces even though the latter may be less expensive in construction and capable of faster conveying speeds. Shuttle-type transfer mechanisms are unique in having lift and carry functions which can be precisely synchronized with the various work cycles of the multiple stations in a processing line to provide, at predetermined precise locations and with split second timing, accurate positioning of the individual workpieces being transferred along the processing line. Thus, by utilizing a shuttle-type transfer line, each workpiece can be intermittently advanced and then held stationary while located precisely at a known position at a known point in time, and the various station work cycles thereby precisely coordinated with workpieces in an intermittent conveying process. This characteristic of shuttle-type transfer mechanisms renders them highly suited for high speed automated processing lines, particularly those where computer controlled and programmed robotic or other automated mechanisms are employed as adjuncts to the work being carried out in the processing line.
For example, high speed repetitive progressive die transfer press sheet metal stamping operations are conventionally employed to impart by progressive stamping the finished curvature and other features into automotive body door panels. The press is operated through its reciprocating work stroke stamping cycle as the blanks are advanced through the progressive dies of the press, and the internal press transfer mechanism and/or a separate press unloader operates rapidly to individually remove each finished stamping one at a time in a very short cycle, which may be on the order of 2 to 4 seconds. Although a typical endless-loop-type conveyor can receive the finished stampings from the press unloader at an operating rate which can keep up with such a rapid work cycle so as to transport a line array of the finished door panels downstream to a gang of unloading stations, the manual operation of unloading the stampings from the downstream end of such a conveyor usually requires a crew of several unloading workmen in order to keep up with the conveyor delivery speed at the conveyor unloading stations.
An automated single robotic unloader has not been available to do this job because its cycle time is too great. The conveyor part unloading cycle requires that the finished stamping be engaged or gripped while on the conveyor, then lifted off of the conveyor, then carried to a storage rack while manipulating the workpiece, usually through a 90.degree. bodily rotation, and then located in a storage slot compartment in a multiple workpiece transit container. After the part is so stored and released, the part unloader must continue to cycle back to the conveyor to pick up the next body panel stamping. The total time of this conveyor unload cycle thus greatly exceeds the rate at which the body stamping workpieces need to be loaded onto and advanced by the conveyor.
Even using a gang of automated robot unloading mechanisms for simultaneously gang unloading a fast moving conventional conveyor does not solve the problem. Generally, robot unloaders have not been commercially developed to a state where they can reliably rapidly find or locate workpieces carried on an endless belt conveyor, even when intermittently operated, much less when the mode of operation produces continuous movement of the workpieces on the conveyor. Rather, to achieve safe, reliable and efficient automated unloading operations, robotic unloading mechanisms need the accurate synchronization provided by a shuttle-type transfer mechanism such that the workpiece stampings are reliably and accurately delivered to an unloading station, held immobile in a dwell phase of the cycle at a precise location at a given point of time in the work cycle, and for a precise predetermined period of time to thereby enable the robot to find, securely engage and lift the workpiece off and out of the unload station.
Although shuttle-type transfer systems and robotic unloaders are thus highly compatible for use in automated processing lines, conventional shuttle-type transfer mechanisms inherently impose another cycle rate limitation. Due to their aforementioned inherent closed loop shuttle mode of travel motion, there is a minimum finite cycle time required for the typical shuttle mechanism to move through its rectangular travel path in a vertical plane to accomplish the sequence of (1) engaging the workpiece, (2) lifting the same, (3) carrying the workpiece on an advance stroke, (4) lowering the workpiece onto a fixed locator, (5) continuing to lower to the clearance position and then (5) moving on its retraction stroke back to the pickup position in its path. The minimum duration of this cycle is limited by such factors as the horizontal and vertical stroke distances needed, the mass of the moving parts of the transfer shuttle mechanism itself and the mass of the total workpiece load being carried by the transfer shuttle, all of which must be respectively accelerated, decelerated and held stationary, the need to smoothly and rapidly transfer workpieces both vertically and horizontally without jarring, shocking or mislocating them, and the limitations of the power drive train components. All of these factors constrain the maximum operating speed and hence minimum cycle time hitherto achievable with a conventional shuttle-type transfer mechanism.
In view of the foregoing considerations and problems, as far as is known it has not hitherto been feasible to utilize shuttle-type transfer mechanisms and associated synchronized automated robotic unloading mechanisms to thereby fully automate high speed workpiece processing lines because the maximum piece loading and/or unloading rate of the transfer conveyor line is not fast enough to keep up with the short cycle, high output rate of such high speed machines.